The article says: "If you inspect “Remove Section,” and change the corresponding act=”z” snippet to act=”n” (highlighted in the image below) and then click “Remove Section,” this will add another new section! There doesn’t seem to be a cap on the number of sections you can create through this technique, and these changes will persist for future browser sessions just like the Unread trick."

I am trying to do this in 2016. I successfully added several more sections to my priority inbox using this method, but I notice that when I do more than 5 sections, the bottom sections start auto-collapsing, and there's no way to actually show the messages in that category in the priority inbox view. You have to go to the right-hand drop down arrow above the section and hit "Show more messages" to see them - and they open in a new screen, which I don't want.

In sum, I can't get this hack to work properly for more than 5 total sections. Any advice?

In 2018 this hasn't changed unfortunately. It looks like the Gmail client is correctly requesting all of the inbox sections that are defined, but the server only ever returns the first 5. The UI is officially limited to 3 sections, so there's probably little reason for the Gmail product team to change this on the server side (if Gmail gets any dev love at all these days).
– StecmanMar 22 '18 at 3:55